PROVENANCE CONTROLS ON THE GEOCHEMISTRY AND RADIOGENIC ISOTOPIC COMPOSITION OF PLEISTOCENE TILLS IN MINNESOTA USA

In Minnesota, Pleistocene tills from four major source regions—Riding Mountain (northwest), Winnipeg (north/northwest), Rainy (north/northeast), and Superior (northeast)—are traditionally differentiated primarily on the basis of matrix color and clast composition. Here we report on an investigation of glacial sediment provenance using a combination of geochemical approaches, including bulk sediment X-ray fluorescence (XRF), multi-element ICP-MS analysis, and Sr and Nd isotopic composition. We analyzed a series of carbonate-rich tills taken by rotasonic core from Renville County MN, as well as a series of reference till samples representing each of the major source regions described above. Glaciolacustrine samples were also examined from key localities. We conducted parallel analyses of <2-mm and <63-µm size fractions, and performed XRF and bulk X-ray diffraction (XRD) on samples before and after carbonate removal. Isotopic and ICP-MS analyses were conducted on carbonate-free samples. Principal component and discriminant function statistical analyses highlighted differences in elemental geochemistry among regionally recognized Pleistocene lithostratigraphic units, but statistical analysis did not show a dramatic difference between size fractions, suggesting that use of either fraction is justifiable. Radiogenic isotopic compositions of the fine (<63 µm) fraction show distinctive signatures consistent with bedrock source regions, and down-core trends in Sr- and Nd-isotopic composition show coherent variation relative to other provenance signatures including trace element ratios (e.g. Zr/Sc) and the chemical index of alteration. Analyzed lake sediment samples are consistent with Nd-isotopic compositions of tills from the same provenance, but trace element and Sr-isotopic signatures show evidence of alteration by processes that may include sedimentary transport or weathering. The rare-earth element compositions of glacial tills show very little relationship with provenance. Overall, our results demonstrate that a multi-element approach to Pleistocene till geochemistry is capable of providing insight into sediment provenance as well as process.